I would like to praise josh3736's answer for providing some excellent historical context. While it's well articulated, the CSS landscape has changed in the almost five years since this question was asked. When this question was asked, px
was the correct answer, but that no longer holds true today.
tl;dr: use rem
Historically px
units typically represented one device pixel. With devices having higher and higher pixel density this no longer holds for many devices, such as with Apple's Retina Display.
rem
units represent the root em size. It's the font-size
of whatever matches :root
. In the case of HTML, it's the <html>
element; for SVG, it's the <svg>
element. The default font-size
in every browser* is 16px
.
At the time of writing, rem
is supported by approximately 98% of users. If you're worried about that other 2%, I'll remind you that media queries are also supported by approximately 98% of users.
px
The majority of CSS examples on the internet use px
values because they were the de-facto standard. pt
, in
and a variety of other units could have been used in theory, but they didn't handle small values well as you'd quickly need to resort to fractions, which were longer to type, and harder to reason about.
If you wanted a thin border, with px
you could use 1px
, with pt
you'd need to use 0.75pt
for consistent results, and that's just not very convenient.
rem
rem
's default value of 16px
isn't a very strong argument for its use. Writing 0.0625rem
is worse than writing 0.75pt
, so why would anyone use rem
?
There are two parts to rem
's advantage over other units.
px
value of rem
to whatever you'd likeBrowser zoom has changed a lot over the years. Historically many browsers would only scale up font-size
, but that changed pretty rapidly when websites realized that their beautiful pixel-perfect designs were breaking any time someone zoomed in or out. At this point, browsers scale the entire page, so font-based zooming is out of the picture.
Respecting a user's wishes is not out of the picture. Just because a browser is set to 16px
by default, doesn't mean any user can't change their preferences to 24px
or 32px
to correct for low vision or poor visibility (e.x. screen glare). If you base your units off of rem
, any user at a higher font-size will see a proportionally larger site. Borders will be bigger, padding will be bigger, margins will be bigger, everything will scale up fluidly.
If you base your media queries on rem
, you can also make sure that the site your users see fits their screen. A user with font-size
set to 32px
on a 640px
wide browser, will effectively be seeing your site as shown to a user at 16px
on a 320px
wide browser. There's absolutely no loss for RWD in using rem
.
px
ValueBecause rem
is based on the font-size
of the :root
node, if you want to change what 1rem
represents, all you have to do is change the font-size
:
:root {_x000D_
font-size: 100px;_x000D_
}_x000D_
body {_x000D_
font-size: 1rem;_x000D_
}
_x000D_
<p>Don't ever actually do this, please</p>
_x000D_
Whatever you do, don't set the :root
element's font-size
to a px
value.
If you set the font-size
on html
to a px
value, you've blown away the user's preferences without a way to get them back.
If you want to change the apparent value of rem
, use %
units.
The math for this is reasonably straight-forward.
The apparent font-size of :root
is 16px
, but lets say we want to change it to 20px
. All we need to do is multiply 16
by some value to get 20
.
Set up your equation:
16 * X = 20
And solve for X
:
X = 20 / 16
X = 1.25
X = 125%
:root {_x000D_
font-size: 125%;_x000D_
}
_x000D_
<p>If you're using the default font-size, I'm 20px tall.</p>
_x000D_
Doing everything in multiples of 20
isn't all that great, but a common suggestion is to make the apparent size of rem
equal to 10px
. The magic number for that is 10/16
which is 0.625
, or 62.5%
.
:root {_x000D_
font-size: 62.5%;_x000D_
}
_x000D_
<p>If you're using the default font-size, I'm 10px tall.</p>
_x000D_
The problem now is that your default font-size
for the rest of the page is set way too small, but there's a simple fix for that: Set a font-size
on body
using rem
:
:root {_x000D_
font-size: 62.5%;_x000D_
}_x000D_
_x000D_
body {_x000D_
font-size: 1.6rem;_x000D_
}
_x000D_
<p>I'm the default font-size</p>
_x000D_
It's important to note, with this adjustment in place, the apparent value of rem
is 10px
which means any value you might have written in px
can be converted directly to rem
by bumping a decimal place.
padding: 20px;
turns into
padding: 2rem;
The apparent font-size you choose is up to you, so if you want there's no reason you can't use:
:root {
font-size: 6.25%;
}
body {
font-size: 16rem;
}
and have 1rem
equal 1px
.
So there you have it, a simple solution to respect user wishes while also avoiding over-complicating your CSS.
I was afraid you might ask that. As much as I'd like to pretend that rem
is magic and solves-all-things, there are still some issues of note. Nothing deal-breaking in my opinion, but I'm going to call them out so you can't say I didn't warn you.
em
)One of the first issues you'll run into with rem
involves media queries. Consider the following code:
:root {
font-size: 1000px;
}
@media (min-width: 1rem) {
:root {
font-size: 1px;
}
}
Here the value of rem
changes depending on whether the media-query applies, and the media query depends on the value of rem
, so what on earth is going on?
rem
in media queries uses the initial value of font-size
and should not (see Safari section) take into account any changes that may have happened to the font-size
of the :root
element. In other words, it's apparent value is always 16px
.
This is a bit annoying, because it means that you have to do some fractional calculations, but I have found that most common media queries already use values that are multiples of 16.
| px | rem |
+------+-----+
| 320 | 20 |
| 480 | 30 |
| 768 | 48 |
| 1024 | 64 |
| 1200 | 75 |
| 1600 | 100 |
Additionally if you're using a CSS preprocessor, you can use mixins or variables to manage your media queries, which will mask the issue entirely.
SafariUnfortunately there's a known bug with Safari where changes to the :root
font-size do actually change the calculated rem
values for media query ranges. This can cause some very strange behavior if the font-size of the :root
element is changed within a media query. Fortunately the fix is simple: use em
units for media queries.
If you switch between projects various different projects, it's quite possible that the apparent font-size of rem
will have different values. In one project, you might be using an apparent size of 10px
where in another project the apparent size might be 1px
. This can be confusing and cause issues.
My only recommendation here is to stick with 62.5%
to convert rem
to an apparent size of 10px
, because that has been more common in my experience.
If you're writing CSS that's going to be used on a site that you don't control, such as for an embedded widget, there's really no good way to know what apparent size rem
will have. If that's the case, feel free to keep using px
.
If you still want to use rem
though, consider releasing a Sass or LESS version of the stylesheet with a variable to override the scaling for the apparent size of rem
.
* I don't want to spook anyone away from using rem
, but I haven't been able to officially confirm that every browser uses 16px
by default. You see, there are a lot of browsers and it wouldn't be all that hard for one browser to have diverged ever so slightly to, say 15px
or 18px
. In testing, however I have not seen a single example where a browser using default settings in a system using default settings had any value other than 16px
. If you find such an example, please share it with me.